a(n) = a(n-1) + 3*a(n-2) - a(n-3) - 2*a(n-4), n > 3.

Terms

a(0) =1a(1) =2a(2) =3a(3) =4a(4) =9a(5) =14a(6) =31a(7) =56a(8) =117a(9) =226a(10) =459a(11) =908a(12) =1825a(13) =3638a(14) =7287a(15) =14560a(16) =29133a(17) =58250a(18) =116515a(19) =233012a(20) =466041a(21) =932062a(22) =1864143a(23) =3728264a(24) =7456549a(25) =14913074a(26) =29826171a(27) =59652316a(28) =119304657a(29) =238609286